1. Field of the Invention
Aspects of the present invention relate to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same. More particularly, aspects of the present invention relate to an electrolyte for a lithium secondary battery that has a long life and that is preserved well and a lithium secondary battery including the same.
2. Description of the Related Art
Recently, as apparatuses such as camcorders, mobile telephones, and notebook personal computers (PC) become smaller and lighter and have higher performances due to the rapid developments of electronic industry, communication industry, and computer industry and electronic products, light and reliable batteries that can be used for a long time are required. In particular, since a rechargeable lithium secondary battery has an energy density per unit weight that is three times higher than the energy densities per unit weight of a conventional lead battery, a conventional Ni—Cd battery, a conventional Ni—H battery, and a conventional Ni—Zn battery and can be rapidly charged, research and development on the rechargeable lithium secondary battery has actively performed worldwide.
Lithium metal oxides are used as the positive electrode active materials of a lithium secondary battery. A lithium metal, a lithium alloy, (crystalline or amorphous) carbon, and a carbon composite are used as the negative electrode active materials of a lithium secondary battery. The term “lithium secondary battery” may refer to a lithium ion battery, a lithium ion polymer battery, or a lithium polymer battery in accordance with the kind of separator and electrolyte that are used. A lithium secondary battery may be a cylinder type battery, a polygon type battery, a coin type battery or other types in accordance with the shape thereof.
The average discharge voltage of the lithium secondary battery is in the range of from 3.6V to 3.7V, which is higher than the discharge voltages of other alkali batteries such as a Ni-MH battery, or a Ni—Cd battery. However, in order to obtain such a high driving voltage, the electrolyte composition used in the battery should be electrochemically stable in the range of from 0V to 4.2V, which is the charge and discharge voltage region. Therefore, an organic electrolyte obtained by dissolving lithium salt in a non-aqueous organic solvent is used as an electrolyte for a lithium secondary battery. An organic solvent having high ion conductivity and dielectric constant and low viscosity is preferably used as the organic solvent. However, since the single non-aqueous organic solvent that satisfies all of the above-described conditions has not yet been found, a solvent obtained by mixing together an organic solvent of a high dielectric constant and an organic solvent of a low dielectric constant or a solvent obtained by mixing together an organic solvent of a high dielectric constant and an organic solvent of low viscosity may be used.
A method of improving the ion conductivity of an organic solvent obtained by mixing together a chain carbonate such as dimethyl carbonate or diethyl carbonate and a cyclic carbonate such as ethylene carbonate or propylene carbonate is disclosed in U.S. Pat. Nos. 6,114,070 and 6,048,637. However, this organic solvent mixture cannot be used at a temperature higher than 120° C. since a gas may be generated by steam pressure at such temperatures such that the battery may swell.
An electrolyte including an organic solvent including at least 20% of vinylene carbonate (VC) is disclosed in U.S. Pat. Nos. 5,352,548, 5,712,059, and 5,714,281. However, since the dielectric constant of vinylene carbonate is smaller than the dielectric constants of ethylene carbonate, propylene carbonate and gamma-butyrolactone, when the vinylene carbonate is used as a main solvent, the charge and discharge characteristics and the high rate characteristic of the battery significantly deteriorate.
On the other hand, in order to suppress the reductive decomposition of a solvent on a negative electrode, a method of adding a compound that forms a solid electrolyte interface (SEI) on a negative electrode to an electrolyte as a means of suppressing the reductive decomposition of lithium on the negative electrode is disclosed in Japanese Patent Publication No. hei 2001-6729. However, when such a film forming additive is used, since a high resistance SEI in which the conductivity of lithium ions is low is formed on the negative electrode, the discharge characteristic of a battery significantly deteriorates. If an excessive amount of film forming additive is added to the electrolyte, when the excessive amount of film forming additive is preserved at a high temperature, the film forming additive may oxidize and disintegrate on a positive electrode to generate a gas so that the battery may significantly swell due to the increase in internal pressure.